go/parser: better error message for unexpected ',' in struct type

Fixes #12437.

Change-Id: I5463970a6259527003eb0e12903a338cc78e0683
Reviewed-on: https://go-review.googlesource.com/14564Reviewed-by: Chris Manghane <cmang@golang.org>

src/go/parser/parser.go

@@ -410,9 +410,14 @@ func (p *parser) expectClosing(tok token.Token, context string) token.Pos {
 func (p *parser) expectSemi() {
 	// semicolon is optional before a closing ')' or '}'
 	if p.tok != token.RPAREN && p.tok != token.RBRACE {
-		if p.tok == token.SEMICOLON {
+		switch p.tok {
+		case token.COMMA:
+			// permit a ',' instead of a ';' but complain
+			p.errorExpected(p.pos, "';'")
+			fallthrough
+		case token.SEMICOLON:
 			p.next()
-		} else {
+		default:
 			p.errorExpected(p.pos, "';'")
 			syncStmt(p)
 		}

src/go/parser/short_test.go

@@ -106,6 +106,8 @@ var invalids = []string{
 	`package p; const x /* ERROR "missing constant value" */ ;`,                            // issue 9639
 	`package p; const x /* ERROR "missing constant value" */ int;`,                         // issue 9639
 	`package p; const (x = 0; y; z /* ERROR "missing constant value" */ int);`,             // issue 9639
+	`package p; var _ = struct { x int, /* ERROR "expected ';', found ','" */ }{}`,         // issue 12437
+	`package p; var _ = struct { x int, /* ERROR "expected ';', found ','" */ y float }{}`, // issue 12437
 }
 
 func TestInvalid(t *testing.T) {